The invention relates to a four cycle outboard internal combustion engine for driving a watercraft with a cylinder housing having cylinders arranged in at least two rows, wherein a piston is reciprocating in each cylinder, said pistons driving an approximately horizontally accommodated propeller drive shaft by way of an approximately vertically arranged crankshaft, each cylinder bank being provided with cylinder head sealing surfaces for a cylinder head housing receiving gas shuttle valves and wherein the cylinder head sealing surfaces of all of the cylinders are arranged in one single cylinder head sealing plane and an exhaust main manifold for all of the cylinders is arranged approximately parallel to the crankshaft in the cylinder head housing in the area of a central plane of the motor which is configured parallel to the crankshaft.
When used as outboard motors, the four cycle internal combustion engines have an advantage over two cycle internal combustion engines which is that a closed lubricant recirculating system may be used, waters and atmosphere being less polluted as a result thereof. On the other side however, it has the disadvantage that the torque of the piston displacement per cylinder is smaller than with the two-cycle internal combustion engine. This torque deficit may be compensated by increasing the number of revolutions and the speed increasing ratio as well as the capacity, the space available with an outboard motor being restricted though.
U.S. Pat. No. 5,704,819 discloses a four cycle internal combustion engine for an outboard motor with two cylinder banks arranged in V-shape. The cylinder head sealing planes are relatively inclined. For each cylinder bank one cylinder head housing is flange-mounted to a cylinder head sealing plane. The fact that each row of cylinders has separate cylinder head sealing planes and cylinder head housings entails the need for relative high expenditure in manufacture and assembly, though. The cylinder banks are inclined to one another at an angle of almost 60xc2x0 so that the construction of the internal combustion engine is relatively wide.
EP 0 654 590 A2 discloses an outboard motor with two cylinder banks that are inclined to one another at a small angle, the cylinders acting upon one unique crankshaft. The cylinder head sealing surfaces of all of the cylinders are arranged in one single cylinder head sealing plane, very simple processing and simple mounting and dismounting of the cylinder head housing being made possible as a result thereof. As all of the cylinders have a uniform cylinder head sealing plane and are arranged in V-shape, the combustion chamber is wedge-shaped which causes asymmetrical load to be exerted on the piston.
The Japanese published application JP 60-161 296 A discloses an outboard motor which is provided with two paralleled motor units that are arranged abreast and whose crankshafts act upon a propeller drive shaft through a common jackshaft, the two crankshafts being provided with the same direction of rotation. The result thereof is a relatively high yawing moment. The fact that the two motor units are realized completely separately prejudices weight and size.
DE 33 22 447 A1 furthermore describes a two-cylinder four cycle internal combustion engine for vehicles with two crankshafts that are rotating in opposite directions. A camshaft for driving the gas shuttle valves of the cylinder head is arranged in the cylinder block between the cylinders, the valves being actuated through a tappet rod and through valve lifters. The camshaft, which is arranged in the cylinder block between the cylinders, bears adversely on the width of the internal combustion engine. Outboard motors of watercrafts require a slim design and that is why this conception cannot be readily adopted.
It is the object of the invention to avoid these drawbacks and to develop a four cycle internal combustion engine for an outboard motor with little expenditure in manufacture and assembly that is light-weighted and small in size and that allows to optimally design the combustion chamber.
The solution to this object in accordance with the invention is to provide one crankshaft for each cylinder bank, the crankshafts having a stable relative speed ratio and acting upon the propeller drive shaft through a common jackshaft. On account of the two crankshafts, there are no restrictions as to the constructional arrangement of the cylinders and to the design of the combustion chambers. A very slim and light design may still be realized.
In order to keep the yawing moment as low as possible it is advantageous to have the crankshafts rotating in different directions. Two cylinders that are positioned in relative neighbourhood relative to the central plane of the motor may thereby execute strokes in synchronism, which permit the realization of a very simple first-rank counterbalance. However, the torque transmitting components may thereby be subjected to larger amounts of strain. In order to avoid this, there may be provided that the cylinders, which are positioned in relative neighbourhood relative to the central plane of the motor, are each provided with an ignition that is offset by at least approximately 90xc2x0, preferably by 180xc2x0.
Another measure for achieving a very small width consists in having the exhaust valves of all of the cylinders controllable by one exhaust camshaft arranged preferably in the cylinder head housing, wherein the exhaust camshaft acts upon the exhaust valves of two cylinders that are positioned in relative neighbourhood relative to the central plane of the motor by way of a valve bridge preferably. Accordingly, three camshafts only are necessary, viz., two intake camshafts and one exhaust camshaft. This camshaft array permits to accommodate in the cylinder head housing at least one intake manifold per cylinder between the central exhaust camshaft and a lateral intake camshaft, which allows to configure a so-called reverse tumble in the combustion chamber. There is more specifically provided that two intake manifolds per cylinder discharge via one intake valve each into the combustion chamber and that at least one exhaust manifold leaves the combustion chamber via an exhaust valve, the intake valves and the exhaust valves being arranged on different sides of a high plane defined by a cylinder axis and an axis of the piston pin, the intake manifolds which are preferably intersecting the high plane being curved in such a manner that a reverse tumble is generated in the combustion chamber, said tumble being oriented from the intake valves toward the piston and from there to the exhaust valve. The exhaust manifold is guided downward relatively centrically relative to the shank of the outboard motor.
As an alternative to the embodiment with the three camshafts, there may be provided that per cylinder bank one camshaft provided with intake and exhaust cams is provided in the cylinder head housing, wherein two like gas shuttle valves per cylinder may preferably be actuated by one single cam by way of a fork rocker arm or a fork drag arm.
In order to keep the width as small as possible it is moreover advantageous to have the axes of all of the cylinders paralleled.
The fuel is preferably supplied by way of an indirect fuel injection device that discharges in at least one intake manifold per cylinder. Direct fuel injection into the combustion chamber is also conceivable though.
In a simple embodiment according to the invention, there is provided that the crankshafts are connected to each other by way of a first toothed gearing and that one of the two crankshafts is connected to the jackshaft by way of a second toothed gearing. The two crankshafts are connected to each other by way of gears that have the same number of teeth. Since the same teeth always mesh together, the tooth flanks may wear at an early stage. In order to avoid this, there is provided in another variant in accordance with the invention that one crankshaft is connected to the jackshaft by way of an encircling transmission and that the other crankshaft is connected to the jackshaft by way of a toothed gearing, the jackshaft preferably driving the camshafts arranged in the cylinder head housing by way of another encircling transmission.
Within the scope of the invention there may also be provided that an automatic transmission with at least two gears be arranged between the crankshaft and the propeller drive shaft.
In another embodiment there may furthermore be provided that the jackshaft is prolonged between the two rows of cylinders up to the front end of the engine opposite the propeller drive shaft. This permits easy drive of auxiliary units, e.g. of a generator and/or a flywheel, located at the front end of the engine opposite the propeller drive shaft by way of the jackshaft.